Chapter 3
The Human Contribution

Photo by: Alx

Scientists know that the earth is warming and most of them acknowledge
that it has warmed about one degree in the past hundred years. What many
of them find alarming is how quickly that change has occurred. Throughout
the planet's history there have been periods of warming and
cooling, but the average rate of change has been about one degree per
thousand years—so the current global warming appears to be
happening ten times faster than ever before. Many scientists believe that
this is the result of anthropogenic actions, which are caused by human
beings rather than nature. Dr. Stephen H. Schneider, biological sciences
professor at Stanford University, is one scientist who believes that
people are to blame for the current global warming, as he explains:

Humans are not simply passengers holding a temporary ticket on planet
Earth's ride through the galaxy. We are actively altering the
surface of the land and the composition of the atmosphere. These factors
affect the natural flows of energy and materials around the planet and
in turn are altering the climate. And while it usually takes nature
thousands of years to create several degrees of temperature change on a
globally
sustained basis, human beings can do so in a century or less.
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Fossil Fuels: Friend or Foe?

Schneider and other like-minded scientists insist that this accelerated
warming is caused by greenhouse gases that humans are adding to the
atmosphere. That is why scientists often use the term
greenhouse warming
to describe the current warming of the earth. Richard Somerville shares
his views on the cause of this increase in temperature: "The
concern is that we human beings are modifying that greenhouse effect by
adding to the atmosphere gases that increase the natural abundance of
these so-called gases. . . . We're adding them through lots of
processes, the most important single one of which is burning fossil fuel
(coal and oil and natural gas), which releases carbon dioxide."
15

Fossil fuels were formed hundreds of millions of years ago from the
fossilized remains of plants and animals. After the organisms died and
decomposed,
they were eventually buried under hundreds or thousands of feet of mud,
rock, and sand. Over time, pressure and heat from the earth compacted the
material into layers of sedimentary rock. Different types of fossil fuels
were formed based on the types of animals and plants that had decomposed,
how long this material was buried, and the degree of temperature and
pressure that existed.

Coal is a fossil fuel that has been used since about 1000
B
.
C
., but during the Industrial Revolution its use began to soar. Since coal
was found to be both plentiful and cheap, it was burned to power the
rapidly growing industry throughout Europe and America. Historian Gale
Christianson describes the pollution of this period: "By the
mid-1800s, the tall chimneys [of factories], each constructed of a million
or more bricks and hundreds of tons of mortar, had far eclipsed the great
cathedrals of medieval Europe, rising as high as 450 feet. They spewed
their burden of gases and effluents high into the atmosphere round the
clock, where it was believed the pollutants would disperse without
harm."
16

During the Industrial Revolution, coal-burning factories like the
one in this drawing contributed a significant amount of carbon
dioxide to Earth's atmosphere.

For many years, coal was considered the primary fuel. It was used not only
for manufacturing, but also for everything from heating homes to providing
power for railroad trains and steamships. Today, coal is used by industry
to manufacture such products as steel and cement, but its leading use is
in making electricity. Power plants use coal to heat water to high
temperatures until it turns into steam, which then rotates large turbines
to create electricity. In the United States, more than 50 percent of all
electrical plants use coal, which also provides power for about 40 percent
of the total electricity generated throughout the world.

The other two fossil fuels, oil and natural gas, are also used to produce
electricity, but not as often as coal. Oil and gas are primarily used to
heat homes and factories, as well as fuel all forms of transportation from
buses to ships and motorcycles to airplanes.

All fossil fuels release carbon whenever they are burned, but coal has a
much higher carbon content than either oil or gas. The Union of Concerned
Scientists says that coal is a main contributor toward global warming
because so much electricity is produced from coal-burning power plants.
The group also says that these power plants are the single largest source
of atmospheric CO
2
. Each year, about 7 billion tons of carbon are released through the
burning of fossil fuels; and when this reacts with oxygen, carbon dioxide
is created—more than 20 billion tons of it.

The Planet's Natural Balance

Carbon dioxide is a colorless, odorless gas that is naturally present in
the atmosphere, but only in tiny amounts. In fact, oxygen and nitrogen
comprise about 99 percent of atmospheric gases, while all the other
gases—including CO
2
—total just 1 percent. Yet even though carbon dioxide is only a
trace gas, it is essential for life. Its powerful heat-trapping
capabilities help keep the earth warm, and CO
2
is also necessary in order for all types of plants to live and grow. That
is because of its role in photosynthesis, the process by which plants
combine light energy from the sun with CO
2
and water to produce their own food.

Carbon dioxide is created naturally when all living things breathe. For
example, the bodies of humans and animals contain about 18 percent carbon.
Each time they inhale they take in oxygen, which mixes with the carbon in
their bodies and is then exhaled as carbon dioxide in a process known as
respiration. CO
2
is also formed naturally when living things die and decompose. The carbon
that has been stored in the body of the plant, animal, or human is
released into the soil over time. Eventually it reacts with oxygen in the
soil and releases carbon dioxide into the air.

Together, all these processes make up a natural system that keeps carbon
dioxide levels in balance. As long as the amount of CO
2
that is added to the air through respiration and decay is the same as the
amount that is taken out, that balance is maintained. However, over the
past hundred years, atmospheric concentrations of carbon dioxide have
increased by about one-third—and in that same period of time the
earth has warmed by about one degree Fahrenheit. Many scientists believe
this is not a coincidence.

According to John J. Berger, carbon dioxide is more responsible for
changing the earth's climate than any other gas, as he explains:

This is because we add more of it to the atmosphere—by
far—than any other. Amazing as it may seem, by adding only a few
hundredths of a percent of it to the air, we change our climate. The
Earth's temperature and the concentration of carbon dioxide in
the atmosphere have risen and fallen together for at least the past
420,000
years—as far back in time as our instruments can probe. . . .
Only in the past 150 years, however, have human actions actually begun
markedly raising the carbon dioxide levels in the atmosphere.
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Berger says that more than 65 percent of the warming that has occurred
over the past century has been caused by the carbon dioxide added to the
atmosphere by humans.

The First Warning

The first scientist to propose that increased carbon dioxide could alter
the atmosphere was a Swedish chemist named Svante Arrhenius. In the late
1890s, he studied the paper that had been written by Jean-Baptiste-Joseph
Fourier about seventy years before. Arrhenius agreed with Fourier about
the role of heat-trapping gases in the atmosphere, and he was intrigued
with the scientist's theory that the earth acted like a giant glass
vessel that trapped and held heat. Arrhenius took the theory one step
further, though. He suspected that humans were causing the gases to
accumulate at a faster-than-normal rate because of the burning of fossil
fuels such as coal. Arrhenius thought it was logical that as more fossil
fuels were burned, more carbon was released into the atmosphere. He
believed this could cause atmospheric carbon dioxide levels to rise
significantly—which, he reasoned, would trap more of the
sun's energy and make the earth hotter. For this reason, he used
the example of a hothouse, or greenhouse, model to describe the warming of
the planet.

In 1895, Arrhenius presented a paper to a prominent scientific group in
Stockholm, Sweden. The paper was called "On the Influence of
Carbonic Acid in the Air upon the Temperature of the Ground," and
it expressed his belief that higher levels of carbon dioxide in the
atmosphere could raise the earth's

temperature. Arrhenius was not alarmed by the potential for global
warming; in fact, he thought that it might be a good thing for the planet.
And even though he predicted that atmospheric concentrations of CO
2
would double, he believed that it would be several thousand years before
this happened.

Revealing Discoveries

During the following years, some scientists explored Arrhenius's
theories about the relationship between carbon dioxide and climate.
However, most scientists paid no attention. There was no way to prove that
CO
2
was building up in the atmosphere because there were no instruments to
measure it; and it was assumed the earth's oceans would prevent
carbon dioxide from accumulating in the atmosphere because they could
absorb gas. Then in the 1950s, Roger Revelle and Hans Suess, both
scientists from Scripps Institute of Oceanography, published a research
paper challenging that belief. They concluded that there were
limits to how much CO
2
the oceans could absorb, and that the excess carbon dioxide produced by
industry and automobiles would remain in the air and eventually warm the
planet. Still, the majority of scientists found this theory hard to
believe. These beliefs did not begin to change until Charles David Keeling
designed and built his manometer. His ability to measure carbon dioxide
levels piqued the interest of other scientists.

The real turning point came during the 1980s. Researchers at the Polar
Plateau in Vostok, East Antarctica, drilled thousands of miles into a
glacier and extracted an ice core that dated back more than one hundred
thousand years. By analyzing air bubbles trapped in the ice, the
scientists were able to confirm that carbon dioxide levels had risen
steadily since the mid-1700s. At that point, more scientists began to pay
attention.

Over the following years, measurements continued to be taken at the Mauna
Loa Observatory, and carbon dioxide levels showed steady increases each
year. By the year 2000, atmospheric concentrations of carbon dioxide had
risen to more than 368 ppm—a 17 percent jump from the 1950s when
the first measurements were taken. These findings meant that not only were
CO
2
levels rising, they were rising fast.

Even scientists who doubt that global warming is a problem admit that
humans have increased the amount of CO
2
in the atmosphere. For instance, Dr. Patrick J. Michaels says that
predictions about global warming have been proven inaccurate because the
earth has warmed at a much slower rate than some scientists said it would.
For this reason, he believes that the global warming issue has been blown
out of proportion. However, he also acknowledges that CO
2
levels have risen and that humans have played at least some role in the
increase, as he explains:

It has been known since 1872 that water vapor and carbon dioxide are the
principal "greenhouse"
gases in the atmosphere, and that increasing their concentration should
elevate the temperature in the lower atmosphere. What has been a subject
of contention ever since, is the amount and character of the warming.
Because of all of the atmospheric greenhouse gases emitted by human
activity, we have progressed to roughly a 60% increase in the equivalent
natural carbon dioxide greenhouse effect.
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Deforestation

Burning fossil fuels is not the only human activity that releases carbon
dioxide into the air. The destruction of the world's forests
creates about 30 percent of all anthropogenic greenhouse gases. Living
trees breathe in
carbon and breathe out oxygen, and when they are cut down, huge
concentrations of CO
2
are released into the air. When trees are burned, the effects are doubly
harmful to the environment.

In many parts of the world, especially in tropical countries such as
Brazil, Indonesia, Burma, and Thailand, rain forests are considered more
of a problem than an asset. In order to clear land for planting crops or
raising livestock, the forests are chopped down and burned, a practice
called slash-and-burn agriculture. The environmental group Rainforest
Action Network says that only about half of the world's forests
that existed a thousand years ago remain today. NASA predicts what will
happen if the destruction of forests is not stopped: "The loss of
tropical rain forest is more profound than merely destruction of beautiful
areas. If the current rate of deforestation continues, the world's
rain forests will vanish within 100 years—causing unknown effects
on global climate and eliminating the majority of plant and animal species
on the planet."
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Cutting down and burning forests releases carbon dioxide and other
heat-trapping gases into the atmosphere. That is not the only damage that
is done, however. The immense green canopies of the world's

forests absorb and store enormous amounts of carbon dioxide through the
process of photosynthesis—so much so that scientists refer to them
as "carbon sinks." By destroying the forests people also
ruin the earth's natural ability to keep the environment in
balance.

Beyond CO
2
—the Perils of Methane

Carbon dioxide accounts for most of the anthropogenic gases in the
atmosphere, but other heat-trapping gases have been building up over time.
Methane comprises about 20 percent of these, and concentrations of it are
increasing at the rate of about 1 percent each year. Also, while methane
is not as plentiful as CO
2
, it is about thirty times more powerful at absorbing heat in the
atmosphere—which means its potential for contributing to global
warming is greater than any other gas.

Methane is released in a number of ways. It is emitted into the atmosphere
during the burning of fossil fuels, and also when forests are burned. It
escapes from the ground during oil drilling and coal mining, and is often
vented into the air to prevent underground mine explosions. Natural gas is
about 90 percent methane, and when natural gas is extracted from the
ground, methane can escape through cracked or leaking pipelines. Methane
is also formed during the decay of garbage. In the United States alone,
about 10 million tons of food waste is disposed of each year by commercial
restaurants and households. When organic materials such as food scraps,
grass and tree clippings, leaves, and wood debris are hauled to landfills,
they are buried. Unlike carbon dioxide, methane forms without oxygen; so
as the materials decompose beneath the ground, methane is eventually
released into the atmosphere.

Many people are surprised to learn that a major contributor to methane gas
is the raising of livestock. When animals such as cattle, sheep, goats,
horses, pigs, and camels eat grass and hay, the food is broken
down in their digestive systems by bacteria. Methane is expelled into the
air when these animals belch, and when their manure decomposes. In
countries where there is a high amount of agriculture, farm animals are
often the largest source of methane. In Scotland, for instance, farm
animals produce more than 45 percent of the country's total
methane, while in New Zealand the number is much higher: nearly 90
percent. Currently, the worldwide cattle population is increasing faster
than the human population, and as more cattle are raised, more methane gas
is created.

Rice agriculture is another leading source of global methane emissions,
and it is responsible for about 10 percent of the anthropogenic methane in
the atmosphere. Rice is the staple food for more than half of the
world's people, especially in Asian countries such as China and
India. As the population of these countries continues to grow, more rice
must be produced. For much of the growing season, rice farmers flood their
rice paddies with water to help control bugs and weeds. Mud-dwelling
bacteria and other organisms break down organic material in the
waterlogged soil to produce methane. Then, as the hollow stems of the rice
plants act as tubes, the methane gas moves up from the soil and into the
air.

Other Heat-Trapping Gases

Besides carbon dioxide and methane, there are other gases that add to the
atmosphere's heat-trapping ability. Chlorofluorocarbons, usually
called CFCs, are created synthetically for use in refrigerators, air
conditioners, foam, and insulation products, as well as for propellants in
spray cans. CFCs are up to sixteen thousand times more effective than
carbon dioxide at absorbing heat, and they contribute about 20 to 25
percent of the total anthropogenic greenhouse gases. Scientific studies
have also connected CFCs with rapid destruction of the ozone layer, a
protective shield of atmospheric gas that absorbs harmful ultraviolet
radiation from the sun. In most parts of the world, including the United
States, CFCs are rarely used because of international control agreements.
However, they have been in use all over the world for more than sixty
years, and some countries still use them today. This is especially
troublesome to scientists because CFCs remain in the atmosphere for at
least one hundred years, and possibly two or three times longer.

Nitrous oxide is a powerful heat-trapping gas that is two hundred times
more heat absorbent than CO
2
and constitutes about 9 percent of the total anthropogenic greenhouse
gases. It is created when fossil fuels are burned, as well as from
slash-and-burn agriculture.
However, the main source of nitrous oxide is chemical fertilizers, which
are usually made from nitrogen. When these fertilizers are spread on lawns
or golf courses, parks or farm fields, nitrogen is released into the soil.
Once it reacts with oxygen in the air, nitrous oxide is formed. In the
past, nitrous oxide was not considered one of the primary heat-trapping
gases. But like carbon dioxide and methane, atmospheric concentrations of
nitrous oxide have continued to increase, and many scientists now believe
it is a contributor to global warming.

The Uncertainty Lingers

For as long as there has been a planet Earth, there have been fluctuations
in temperature and this will continue in the future. Solid evidence has
proven this to be fact, and it is the one area where all scientists agree.
Where they differ is over the issue of human
activities—specifically, whether or not anthropogenic greenhouse
gases are causing the earth to warm at an unnaturally fast pace, and how
much this will affect future climatic conditions. Scientists who believe
humans are to blame say the evidence speaks for itself—that in the
past several hundred years, humans have altered the atmosphere so much
that irreparable damage has been done. Somerville sums up why he and other
scientists have made these conclusions:

Climate has varied on every time scale to which we have any
observational access. Ice ages come and go on time scales of tens of
thousands of years, for example. . . . Climate changes. It changes on
all time scales. What's different between our time and our
grandparents' time is that now humankind, which has been a
passive spectator at this great natural pageant, has become an actor and
is up on the stage. And what we—all 6 billion of us—do can
affect the climate.
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User Contributions:

couldn't global warming be caused by the tremendous heat in the core of the earth rising to the surface because of the extraction of oil from the earth? The oil in the earth is for a purpose, and it is unlikely that purpose is for us to burn in vehicles. more likely the oil is in the earth to insulate the surface of the earth from the heat generated in the core of the earth. the more oil that is extracted from the earth the lower the "R" value between the core of the earth and the surface.

The content is very true and resourseful for research work. In the near future we scientists should come up with new methods of reducing this carbon dioxide gas produced by the most of industries to carbon which is not very harmful to atmosphere and environment as well

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